1. Field of the Invention
The present invention relates to satellite communications systems that provide low-cost, over the satellite telephony connections to rural and other previously non-served areas. Communication is supported through existing, non-processing satellites to connect very small aperture terminal (VSAT) stations at remote sites to gateway terminals for connection to the public switched telephone network (PSTN).
2. Description of the Related Technology
There is a need to provide quality communication services to many groups of service users that are classified as remote (rural and suburban), primarily in emerging countries that are lacking a solid telecommunications infrastructure. In these emerging or developing countries there is insufficient telecommunications infrastructure to large portions of the population plus, where infrastructure does exist, it inadequately services the attendant population. These telecommunication services include voice (telephony), facsimile and data communications. These telecommunications services can be readily supplied through the use of satellite repeaters to relay digitized telephony signals from a large number of remote terminals/subscribers to one or more gateway terminals for interconnection to the public switched telephone network (PSTN).
Traditionally, communications between a satellite terminal, such as a VSAT, and a hub within a satellite-based network of the type addressed by this invention are performed in a TDM/TDMA (Time Division Multiplex/Time Division Multiple Access) or TDM/CDMA (Code Division Multiple Access) manner, where the outbound channel from the hub to the VSAT is operated as TDM, while the inbound channel can be TDMA or CDMA. TDMA requires higher transmission rates and more VSAT transmit power and is not a cost-effective solution for rural telephony applications. CDMA, on the other hand, uses spread-spectrum techniques to enable multiple VSATs to share a common bandwidth while reducing the VSATs transmit power level requirements. The inbound channel (VSAT to hub) is normally operated with the VSATs transmitting asynchronously, where the individual VSAT signals can arrive at the hub non-time aligned. When the channels are operated non-time aligned, the full cross-correlation properties of the codes are diminished such that (1) a long code is required (which corresponds to a larger transmission bandwidth) and (2) the number of terminals that can access the bandwidth is reduced because of the amount of multiple access interference. As such, CDMA is less efficient in terms of bandwidth for the fixed satellite channel application than other multiple access techniques such as TDMA or Frequency Division Multiple Access (FDMA). A special class of CDMA, referred to as synchronous CDMA, offers improved bandwidth efficiency by aligning the received symbols at the receiver. When the symbols are aligned, the cross-correlation benefits of the codes can be better realized such that shorter codes may be employed and more terminals can access the given bandwidth. In practice, these codes are normally selected from a family of codes known as Gold codes, due to the good cross-correlation properties exhibited by this code set, where a unique code is assigned to each VSAT. While this technique offers good multiple access performance, a bank of receivers are required at the hub to demodulate the inbound transmissions, where each receiver is matched to one of the assigned pseudo-noise or pseudorandom (PN) codes.
The prior art is replete with techniques for providing multiple users with access to a common communication channel. Division by time (TDMA) or frequency (FDMA) are common methods. These techniques generally provide perfect separation among users and therefore provide close to 100% network efficiency. Another technique obtains division by a pseudo random code. A variant of this method is CDMA where each user is assigned a unique code sequence as described by A. J. Viterbi, CDMA: Principles of Spread Spectrum Communication, Addison Wesley, 1995. Collectively, the different codes are sufficiently different, or orthogonal, such that a receiver matches to and only receives transmissions associated with its code of interest. Transmissions associated with other codes are rejected. This can only occur exactly when the users are mutually synchronized and their codes are perfectly orthogonal.
In digital, spread-spectrum communication, each information bit, obtained from a digital source and taking on the binary numbers of either 0 or 1, is transformed into a sequence of N binary numbers, where N is an integer. This transformation is called spreading. Specifically, let c.sub.1. . . c.sub.N be the sequence of binary numbers having some desirable correlation property and let d.sub.k be an information bit. The N binary numbers resulting from this transformation are obtained by the Exclusive-Or of d.sub.k with each c.sub.1. . . c.sub.N. Each individual binary number c.sub.j is called a chip and collectively they are called a spreading code or simply the code. Since there are N chips/bit and denoting the duration of a bit as T.sub.b then the duration of a chip is T.sub.c =T.sub.b /N. Since the baud interval of the transmitted signal has been reduced by a factor of N, the corresponding transmitted waveform has a bandwidth expansion of N, hence, the term spreading. During the process of modulation, the logical values of 0 and 1 are mapped to the real numbers +1 and -1.
When considering multiple access systems which employ spread spectrum, the notion of synchronization between the users is of fundamental importance. There are three ways to define the relative synchronous relation between users. First is asynchronous, which implies no relative timing relationship among the users. Secondly, there is chip synchronization, wherein the chip boundaries of duration T.sub.c align exactly. The information bits of the two users are not in alignment. Thus, chip synchronous does not imply bit synchronous. This leads to the third defining relationship which is bit synchronous. Here, the information bits of each user operating in the multiple access system are in alignment. In particular, the boundaries of duration T.sub.b defining the bit duration for each user align exactly. An example in this case is given in FIG. 1. Clearly, bit synchronism includes chip synchronism.
FIG. 1 illustrates the synchronous relationship between spread transmissions 102 of different users, e.g., user 1 (104) and user k (106), where each user has a different spreading sequence 108 denoted C.sub.k (1) . . . C.sub.k (N) for the k.sup.th user. If synchronous conditions do not exist, then multiple access interference (MAI) results which degrades communication performance. MAI can be mitigated by increasing the length of the code sequence. Of the two effects, synchronization plays the dominant role with regard to MAI. In particular, asynchronous operation even with perfectly orthogonal codes incurs substantial interference losses. When synchronous, a system can still perform well even with small levels of cross correlation. The requirement that each user possess a unique code presents additional difficulties. The network is generally required to support a modest number of users. Hence, existence of large families of codes with minimum cross correlation is essential. Additionally, unique codes greatly increase the complexity at a hub or gateway where transmissions are being received from a large number of users. In this situation, the hub must possess separate receivers for each active user.
A technique which circumvents this complexity issue is spread slotted ALOHA as described by M. K. Simon, J. K. Omura, R. A. Scholtz & B. K. Levitt, Spread Spectrum Communications Vol. III, Computer Science Press, 1985, and by N. Abramson, "VSAT data networks," Proc. IEEE, vol. 78, no. 7, pp. 1267-1274, July 1990. FIG. 2 illustrates the (chip) synchronous relationship 110 between the users, e.g., user 1 (112) and user k (114). With N chips/bit, the users can be chip synchronous when any of the N chips of one user align with any of the N chips of another user. In FIG. 2, the first chip of user k (114) is aligned with the k.sup.th chip of another user 112. It is clear from this figure that the information bits of the two users are not in alignment. Here, each user is chip synchronous but not bit synchronous. Also, each user employs the same spreading sequence C(1) . . . C(N) 118. The sequence 118 is chosen such that the sidelobes in its autocorrelation function are minimal. Multiple access can then be obtained by restricting the relative transmission times between users to be an integer multiple 116 of a chip time. Note in particular that each user is utilizing the same spreading sequence 118 and that their transmissions are not bit aligned. Since each user has the same spreading sequence 118, the reduction in complexity is due to the elimination of multiple matched correlators. In particular, at a hub or gateway, a single correlator receiver can demodulate multiple user transmissions since the detected information from each user appears at the correlator output separated in time by the relative transmission time.
MAI occurs because of the autocorrelation sidelobes. The MAI is compounded since the users are not bit synchronous. In particular, the sidelobe levels of the autocorrelation function do not completely describe the MAI with this technique. The actual sidelobe level in the transmitted signal also depends on the user's bit patterns. Referring to FIG. 2, one bit duration from user 1 will generally overlap with two bits of any other user. For example, a first bit (not shown) for user k (114) ends at a time 117 and a new bit 119 begins, such that a latter portion of the first bit and the beginning portion of the new bit 119 for user k overlaps bit 113 of user 1 (112). So when another user incurs a bit transition, the sidelobe levels increase, causing interference for user 1. Consequently, the code sequence chosen in this approach must have sufficient length to mitigate a moderate level of MAI. In contrast to synchronous CDMA, spread slotted ALOHA will not be able to support N active users with an N length code sequence.
U.S. Pat. No. 5,537,397 to Abramson introduces a spread slotted ALOHA technique whereby each VSAT uses the same PN sequence for inbound transmissions to reduce the complexity of the hub. With a single PN sequence, only a single correlator based receiver is necessary at the hub to despread all VSAT transmissions. This system operates with all transmissions synchronized at the chip level. Being non-bit synchronous, data polarity on the individual VSAT transmissions affects the overall multiple access interference level and thereby reduces the multiple access efficiency. For slotted ALOHA applications, where the theoretical throughput capacity is 36.8%, the multiple access capability of this technique is sufficient. Alternative techniques are necessary for demand assigned voice applications, as related to this invention, where 100% utilization is required.
What is needed is a new modulation format for inbound communications between a group of remote VSATs and a centralized hub by means of a non-processing satellite, wherein a new form of CDMA is employed such that the VSAT transmissions are spread using sequences that are assigned from a set of cyclicly related PN sequences. When operated in a bit synchronous fashion, the full cross-correlation benefits of the codes would be maintained for best multiple access performance, while the cyclic relationship between the independent sequences would enable simplification of the gateway receiver implementation.
What is desired is a spread communication system that provides cost effective telephony services to remote and rural subscribers. Cost effective operation implies (1) low cost VSAT equipment, (2) efficient use of satellite resources, and (3) low cost hub equipment.
It would be an advance in VSAT communications to achieve spread-spectrum inbound communication that has little interference between users, for good multiple access performance and best use of available satellite resources, that operates with a low flux density to enable use of small, low cost antenna systems at the VSAT, and that has a waveform structure that inherently reduces the cost of the hub/gateway equipment.